The importance of wearing seat belts has resulted in the development of switches or sensors that indicate that a seat belt is fastened. The output of the switches or sensors can be used to remind or require the seat occupant to fasten a seat belt. More sophisticated vehicle safety systems attempt to maximize vehicle occupant survival, and minimize vehicle occupant injuries, by taking into account as much information as possible about the vehicle occupant. With the development of these more sophisticated vehicle safety systems it has become even more important to be able to determine whether a seat belt is fastened or not so this information can be used in the actuation of airbags and other safety devices. If a vehicle occupant refuses to wear a seat belt, it is possible to take that into account in the airbag's deployment strategy to the benefit of the unbelted vehicle occupant. Because wearing a seat belt confers such a large safety benefit to the vehicle occupant, vehicle systems are often designed to encourage seat belt use by, for example, sounding an alarm until the seat belt is fastened.
To defeat systems designed to encourage seat belt use, vehicle occupants are known to fasten seat belts behind the occupant. In the case of seat belts which must be not only fastened but protracted to turn off a seat belt alarm, occupants are known to fasten the seat belt around the back of the vehicle seat. Seat belts may also be worn only partially with the shoulder belt or the lab belt being placed behind the vehicle occupant. Seat belt sensors that monitor the tension on the seat belt are known. Seat belt tension is monitored principally in order to be able to accurately determine the seat occupant's weight. For example, the deployment of an airbag may be disabled if the seat occupant is below a selected weight based on safety system logic that the seat occupant must be a child, a package, or a child seat. For a child seat, however, the seat belt is tensioned which will press the child seat downwardly causing occupant weight sensors to reflect higher loads. By subtracting the load caused by seat belt tension as measured by a seat belt tension sensor the true weight of the seat occupant can be determined and safety system logic can operate on the basis of the true weight of the seat occupant.
During an actual crash, the loads imposed on the seat belt are quite high as the seat occupant is driven by accelerations many times the force of gravity against the restraining belts. In order to reduce the loads experienced by the vehicle occupant during a crash seat belt retractors have been designed to incorporate energy dissipation mechanisms that allow the seat belts to extend under load, thus absorbing energy.
In a conventional seat belt retractor, an inertial sensor is used to stop the rotation of a spool to prevent the protraction of the seat belt webbing during a crash. In a seat belt retractor with load limiting capabilities the seat belt retractor permits the controlled protraction of the seat belt by incorporating within the retractor an energy absorption (or dissipation) mechanism which permits the spool to rotate after the retractor has been initially locked up. In the past these mechanisms have included crushable bushings (U.S. Pat. No. 5,547,143), deformable tubes (U.S. Pat. No. 3,881,667) or torsion bars (U.S. Pats. Nos. 3,741,494 and 6,012,667 which are incorporated herein by reference). Linear operating load limiters are also known to which a seat belt is anchored or over which the seat belt webbing passes (U.S. Pat. No. 6,145,881 which is incorporated herein by reference).
Operation of a load limiting/energy-absorbing mechanism is a clear indicator that a seat belt is properly fastened about a seat occupant. What is needed is a mechanism and method for determining when a seat belt is not properly fastened about a vehicle occupant during a vehicle crash. Particularly needed is a system based on predicting the operation of the load limiting/energy-absorbing mechanism and determining when it should, but has not, operated, so the lack of a properly fastened seat belt for a vehicle occupant can be determined. Information indicating the lack of proper seat belt use can then be used as an input to the vehicle safety system which determines how best to deploy other safety system components such as airbags.